The present invention relates to an improvement in a drive control device for a vehicle having a continuously variable transmission designed to continuously control a speed ratio defined as the ratio of rotational speed of an output shaft of the continuously variable transmission over the rotational speed of an input shaft thereof.
In the past, belt-driven continuously variable transmissions have been used as automatic transmissions for vehicles. In general, these continuously variable transmission mechanisms have V-shaped pulley devices, each including a stationary pulley and a movable pulley which cooperate to create an effective diameter which is variable. Hydraulic servo devices are used to move the movable pulley. The V-shaped pulley devices are provided on input and output shafts with a driving belt extending therebetween so that rotation of the input shaft can be transferred to the output shaft. Normally, an oil flowrate to the hydraulic servo device on the input side is adjusted by a flow control valve, whereby the effective diameter of the V-shaped pulley device on the input side is changed. The hydraulic pressure of the hydraulic servo device on the output side is varied by a pressure control valve to thereby follow the change of the effective diameter of the V-shaped pulley device on the input side, so that the driving belt does not slip in transmitting the torque.
As compared with an automatic transmission mechanism consisting of a torque converter with groups of planetary gear units, the above-described continuously variable transmission mechanism is advantageous in that it permits abrupt changes in driving force during running of the vehicle, shift shocks are low, and the fuel consumption rate is good. In recent years, demand has increased for further improvements in continuously variable transmission mechanisms.
However, all of the techniques in the prior art yet have some problems to be improved.
Japanese Patent Laid-Open No. 57-161346 (corresponding to U.S. Pat. No. 4,543,077), for example, discloses a technique of controlling the speed ratio of the continuously variable transmission so that an engine speed corresponds to a throttle opening or a vacuum value of an intake manifold. In this method, the speed ratio is controlled in consideration of engine operating conditions (optimum fuel consumption line that minimizes a fuel consumption rate, for example). Therefore, there is a problem that a driver's will is hard to represent especially at a transitional shifting state of the vehicle. For instance, even when the driver increases a depression rate of an accelerator pedal from 10% to 20% to desire that a vehicle driving torque is increased from 10 kgm to 50 kgm, such a desired driving torque of the vehicle cannot be obtained because an engine output depends on the above-mentioned engine operating conditions which are predetermined. Accordingly, it is difficult to make drive control characteristics follow an acceleration performance desired by the driver.
There is disclosed another method in Japanese Patent Laid-Open No. 58-39870 (corresponding to U.S. Pat. No. 4,515,040). Under a normal running condition (non-shifting condition), the vehicle runs on the optimum fuel consumption line. While under a transitional shifting condition, since the response of shifting of the continuously variable transmission is slower than that of a throttle actuator, an actual output follows a broken line as shown in FIG. 11, resulting in isolation from the optimum fuel consumption line to improve the running property as compared with the previous method of Japanese Patent Laid-Open No. 59-32642 (corresponding to U.S. Pat. No. 4,458,561). However, the engine operation shown by the broken line in FIG. 11 is obtained at the transitional shifting state, but an output of engine at this state cannot be arbitrarily controlled. Consequently, it is difficult to set the output to a desired value at the transitional shifting state.